I haven't reviewed the fastboot for mac/windows source code but I assume it contains some sort of magic to figure out what device the kindle is using.

Most of the command examples I've seen seem to just have fastboot cmd something with no indication of device location or name.

thanks!

If you can see the kindle as a USB drive, you can save a file ENABLE_DIAGS on it and reboot to diagnostics mode. In that menu, there is an "Exit" option that contains "Install fastboot package" (or something like that). That enables fastboot, which changes the kindle USB VID/PID to 0x1949/0xD0D0, which is the USB device that fastboot waits to see.

If you cannot get to diags, then you can go directly to fastboot mode with MfgTool (for windows), but fastboot mode will drop out if the USB connection gets disconnected or reset.